Research On Remote Sensing Emergency Monitoring Method Of Lake Area In The Yangtze River Basin Under Extreme Drought Events

In 2022,the Yangtze River Basin will experience the most serious severe drought event since the complete meteorological observation records were available in 1961.The rare phenomenon of "flood season anti-drying" will occur,and the water area of Poyang Lake and Dongting Lake will decrease sharply,which has attracted widespread social attention.In response to the sharp decrease in the area of lakes in the Yangtze River Basin due to the severe drought in 2022,emergency monitoring using remote sensing technology can quantitatively analyze changes in lake area and spatial distribution.The thematic maps produced can provide data support for disaster prevention and mitigation of severe drought events.Therefore,it is very necessary to carry out remote sensing emergency monitoring of lake area in the Yangtze River Basin under severe drought events.This paper takes September 1,2022 as the start time of the virtual emergency monitoring task,and studies the remote sensing data acquisition,lake water extraction and monitoring product release involved in the emergency remote sensing monitoring method.First,use the China Lake Dataset(1960s-2020)to find out the background data of lake area in the Yangtze River Basin.Then,the availability of Landsat8/9 satellite images was analyzed,and the accuracy and efficiency of lake water area extraction by decision tree method and MNDWI were compared.Finally,MNDWI was used to extract the lake water area under the virtual emergency monitoring task time,and compared with the data of 2006 when the extreme drought occurred and the data of 2020 when the basin flood occurred,and the thematic map of lake area change distribution was released.The main research results of this paper are as follows:(1)Background analysis of lake area in the Yangtze River Basin.Statistical analysis of the number and area of lakes in the Yangtze River Basin from 1960s to 2020 shows that the number and area of lakes in the Yangtze River Basin vary in different years.Statistical analysis of the temporal and spatial changes of four types of lakes,including increase,decrease,disappearance and new formation,found out the location of lakes with area changes.Based on the above analysis,the distribution map of interesting lakes in the Yangtze River Basin was made,and the minimum download range of Landsat8/9 satellite images was extracted.(2)Availability analysis of Landsat8/9 satellite images.The analysis is carried out from the three indicators of Landsat8/9 image processing time,cloud cover and coverage in the land area.According to statistics on the image processing time for the whole year of 2022,more than 20%of the image processing time is 2 days,and more than 90%of the image processing time is 16 days.In the whole year of 2022,the cloud amount in the land area of the image is the most in the 0-5%range,and the cloud amount is the least in the 20-25%range.Under the condition of cloud cover below 30%,the appropriate time scale for Landsat8/9 satellite images to be used in emergency monitoring is 32 days.In addition,the amount of images to be downloaded for the task is about 100G,and it takes about 3 hours for multiple computers to download in parallel,which has little impact on the timeliness of the monitoring task.(3)Analysis of the accuracy and efficiency of lake water area extraction.In terms of accuracy,the accuracy of the decision tree method is related to the number and distribution of training samples.In the process of MNDWI extraction,the threshold needs to be determined manually.Both the decision tree method and MNDWI water body extraction methods have relatively high accuracy.In terms of efficiency,MNDWI outperforms decision tree methods.Therefore,MNDWI is preferred for the timeliness of emergency monitoring,and the decision tree method needs to be further studied in the future.(4)Analysis of emergency monitoring results.The number of lakes under the virtual emergency monitoring task time is 506,with a total area of 12911km2.Comparing the data in 2020 and 2022,the total area of lakes in the Yangtze River Basin is 16294km2 and 12911km2 respectively,with an area reduction of 3384km2 and a reduction ratio of 20.8%.The total number of lakes decreased from 515 to 506,with a net decrease of 9,or a reduction of 1.7%.Comparing the data in 2006 and 2022,the total area of lakes in the Yangtze River Basin is 12,699km2 and 12,911km2 respectively,and the area has increased by 212km2,accounting for 1.7%of the increase.The total number of lakes increased from 475 to 506,with a net increase of 31,an increase of 6.5%.Based on the above data,the distribution map of lake area change in the Yangtze River Basin in 2022 and 2020 and the distribution map of lake area change in the Yangtze River Basin in 2022 and 2006 are released.